DESCRIPTION (Verbatim from the application): In the hind limb unweighted (HU) rat model of bed rest and microgravity, HU results in a central movement of body fluid, diuresis and natriuresis, resulting in a decrease in plasma volume. We have observed that rats increase ingestion of saline solution during 24h HU. Further, this ingestion of saline inhibits the decrease in plasma volume normally observed in 24h HU rats. The studies proposed examine the integration of neural and humoral factors influencing sodium ingestion in the HU model of bed rest. The specific aims of the study are: Specific aim 1: Investigate the role of the renal nerves in promoting sodium intake during 24 h HU. Specific aim 2: Investigate the role of hormones in promoting sodium intake during HU. Specific aim 3: Examine the role of the cardiac afferent nerves in regulating the intake of sodium. Specific aim 4: Compare short term (24 h) and long term (14 d) effects of HU on sodium ingestion and plasma volume. We hypothesize that increases in renal afferent nerve activity is critical to HU-induced sodium ingestion. Furthermore, elevated aldosterone may contribute to the ingestion of sodium while stimulation of cardiac afferent nerves may inhibit sodium intake in this model. Integration of these signals by the central nervous system results in a modest sodium intake and maintains plasma volume during HU. During prolonged HU, adaptations may occur which further influence both the ingestion of sodium and blood volume. Understanding bed rest and microgravity-induced changes in body fluid balance requires understanding such factors as the acquisition of solutes. It is hoped that these experiments will contribute to an understanding of both the regulation of body fluid during prolonged bed rest and the problems associated with return to normal posture. Additionally, these data may contribute to understanding of health problems associated with an aging population and with cardiovascular disease such as congestive heart failure.